It has to get a distance reading from somewhere if it is giving you a population number....otherwise it would only be telling you seed flow with no regard for distance and it could not calculate population per acre.

Sometimes monitors will lose that calibration and go back to default which may or may not be accurate. My guess is that is what has happened to you. If it was me, and I was sure the sprockets on the planter were set correctly for the pop I wanted, I would keep planting as long as the monitor said seed was going out. If you know your field sizes, and the number of bags doesn't come out, adjust you planter sprocket settings. That is all anyone did up until not-that-many years ago.

If you have some downtime due to weather, try to recalibrate your monitor. If that's it, it will fix it. If it's not, you'll know and you can chase down the next thing.

90% or more of the older monitors are Dickey-John, whether or not they carry that brand name.